1. Field of the Invention
This invention relates generally to a chemical vapor deposition (CVD) process for the fabrication of hybrid electrodes in microelectronic device structures such as ferroelectric random access memory (FeRAM) device structures.
2. Description of the Related Art
Ferroelectric random access memories (FeRAMs) depend on the use of high integrity ferroelectric materials as a critical component of cell architecture. Although a wide variety of ferroelectric materials have been developed having actual demonstrated and potential utility in device applications, the practical utilization of such materials in device architectures requires that such materials can be readily fabricated in an efficient and cost-effective manner with respect to their integration in conformations including barrier regions, isolation structures, interconnects, vias and electrodes.
The electrical properties of ferroelectric materials in many instances are highly influenced by the materials used to fabricate electrodes for the ferroelectric material structures. For example, ferroelectric materials such as Pb(Zr,Ti)O.sub.3 (PZT) show a strong dependence on the type of electrode used as the contact for the device structure.
Noble metals such as platinum that have a high work function may be used to fabricate electrodes that produce device structures with low electrical leakage characteristics, but the rate of loss of remanent polarization in the structure with repeated switching cycles (ferroelectric fatigue) is unacceptably high.
Conversely, conductive oxide electrodes improve fatigue behavior, but usually at the cost of unacceptably large increase in electrical leakage. Kingon, et. al., (U.S. Pat. No. 5,555,486) and Al-Shareef, et. al. (J. Appl. Phys. 77 (5), Mar. 1, 1995: 2146), the disclosures of which are hereby incorporated herein by reference in their entirety, have demonstrated a hybrid electrode, comprised of dual layers of Pt and conductive oxide, either with the Pt layers on the exterior or the conductive oxide layers on the exterior, and the other electrode material on the interior against the ferroelectric material. When used with such hybrid electrodes, PZT displays both good fatigue endurance and low electrical leakage.
The layered structures of the above-described art represent one approach to achieving the benefits of conductive oxides and Pt electrodes.
An alternative approach utilizes alloyed electrodes, also demonstrated by Kingon, et. al., (U.S. Pat. No. 5,555,486) and Al-Shareef, et. al. (J. Appl. Phys. 77 (5), Mar. 1, 1995: 2146). Alloyed electrodes of Pt and RuO.sub.2 are formed by co-deposition of metals using ion-beam sputtering. In the preferred embodiment the noble metal is alloyed with an element that is relatively immiscible, and that has a tendency to oxidize to an electrically conductive oxide. Examples of such a combination include Pt and Ir where Ir can be oxidized to IrO.sub.2. Other examples of alloy systems with a high degree of immiscibility include Pd--Ir, Rh--Ir and Pd--Ru. In the latter example, Ru oxidizes to RuO.sub.2. Still further examples include mixtures of noble metals (e.g., Pt, Pd, Rh) and complex conductive oxides (LaSrCoO.sub.3, SrRuO.sub.3, indium tin oxide, and yttrium barium copper oxide). The bi-phasic microstructure consists ideally of noble metal and oxidizable metal grains uniformly distributed. The oxidizable phase may be oxidized during the co-deposition process as taught by Kingon, et. al., and Al-Shareef, et. al., by a separate anneal in an oxidizing ambient (e.g., O2, N.sub.2 O, ozone) before deposition of the ferroelectric, or alternatively, during the deposition process for the ferroelectric which is usually carried out in an oxidizing ambient.
As device density increases, a need arises for conformal coatings to cover sidewalls and to fill small features with electrodes. Wet methods of deposition are have poor conformality, as do line of sight processes such as evaporation. Sputter deposition offers a higher degree of conformality, but composition control of complex oxides is difficult.
Accordingly, there is a need in the art for improved electrode fabrication techniques to maximize the benefits achievable with composite electrode structures.